350- Screwed Stays. 1 holding up may be obtained as at 34 or 35, by pressing on the levers there shewn. But the boiler may be finally closed by machine, using the methods at p or M, Plate XVI. The former is adapted to internally flanged boilers, the tube plate being cut in three pieces at the stiffening plates. After the flange has been riveted, the various tube plate rivets may be closed by the usual Lever Riveter with long arms, dropped in through the furnace holes. The best result is obtained by a boiler designed as at M, Plate XVI., and this should be employed whenever the ship designers permit it. The 'Combustion Chambers are put together as at 39 and 40, but the back plates are riveted by hand, with rivets like 38, unless the flanges be made as at M, Plate XVI. The chambers and furnaces are next put within the boiler shell, and the latter closed They are slung as at 41, carefully blocked and bolted in position, then clamped at the front. Placing the boiler on a cradle, before horizontal drills, and on the machine in Plate XIII., drill the stay holes through into the Combustion Chamber to ensure exact alignment for the screw threads. All stay holes, including those between the chambers, are now tapped, as at 42, by a tap whose threads a and b are continuous. The Screwed Stays are prepared on the machine at 43. Stay a is coupled to spindle £, which revolves by gearing c; screw b has the same pitch as die nut d, and prevents the forma- tion of unequally pitched threads on the stay by 'drawing' or uneven pressure. The stays, having a square on their ends, are now placed in the boiler with a wrench, a nick being first turned at each end to represent their exact lengths; so that having been advanced to correct position, a sharp twist will break off the surplus material. Nats are now added, and the stay ends trimmed up. The boiler being still upon its cradle, the rivet holes at the furnace mouth are set out and drilled by the machine at 44. The drill bracket may be revolved on a horizontal axis by worm gearing, and this, coupled with the rotation of the boiler, will enable us to drill all round. The riveting-up is shewn at K, Plate XVI.